The invention relates to photodiodes used in optical network receivers. In particular, the invention relates to photodiodes having a unipolar or uni-traveling-carrier structure.
Semiconductor photodetectors, most notably various forms of photodiodes, absorb incident light in the form of photons and convert the absorbed photons into an electric current. The current within a lattice of the semiconductor is often represented in terms of xe2x80x98free carriersxe2x80x99 or simply xe2x80x98carriersxe2x80x99. In particular, when a photon with sufficient energy interacts with an atom of the semiconductor lattice, an electron associated with the atom moves across an energy band gap from a valence shell or band to a conduction shell or band of the semiconductor. Movement of the electron across the band gap creates a negative carrier, i.e., the electron, and leaves behind a positive carrier known as a xe2x80x98holexe2x80x99. After carrier generation through photon absorption, a carrier transport mechanism within the semiconductor-based photodetector separates the generated holes and electrons, thereby creating an electric current known generally as a photocurrent. In general, both the electron and the hole may act as carriers within the semiconductor and contribute to the photoelectric current. The photocurrent thus created enables the photodetector to interact in various ways with an external circuit or system. Among other things, photodiodes find wide-scale application in optical receivers used for optical communication networks.
Photodetector performance is often summarized in terms of bandwidth, efficiency, maximum current output, and optical wavelength range. Bandwidth is a measure of a speed of response of the photodetector to changes in an incident optical signal or light source. Efficiency measures how much of the incident optical signal is converted into carriers. Maximum current output is typically determined by a saturation condition within the semiconductor of the photodetector while optical wavelength range is a function of certain material properties of the photodetector among other things. In general, photodetector performance is limited by a combination of material properties of constituent materials of the photodetector and a structural characteristic of the photodetector associated primarily with a type and/or structure of a given photodetector.
For example, FIG. 1A illustrates a cross section of a conventional positive-intrinsic-negative (PIN) photodiode 10. The PIN photodiode 10 comprises an intrinsic or lightly doped semiconductor layer 14 (i-layer) sandwiched between a p-type semiconductor layer 12 (p-layer) and an n-type semiconductor layer 15 (n-layer). The i-layer 14 is often referred to as a photo-active or a light absorption layer 14 since ideally, photon absorption is primarily confined to the i-layer 14 of the PIN diode 10. Typically a deposited metal, such as aluminum (Al), or another conductive material, such as heavily doped polysilicon, form a pair of ohmic contacts 17a, 17b, that provide an electrical connection between the PIN photodiode and an external circuit. The ohmic contact 17a connected to the p-layer is called an anode contact 17a while the ohmic contact 17b connected to the n-layer is referred to as a cathode contact 17b. Typically, the PIN photodiode 10 is formed on and structurally supported by a semi-insulating substrate 19.
FIG. 1B illustrates a band diagram 20 of the PIN photodiode 10 illustrated in FIG. 1A. The band diagram 20 depicts energy levels as electron-volts (eV) in a vertical or y-direction and physical length or distance along a conduction path within a device in a horizontal or x-direction. Thus, the band diagram 20 illustrates a valence band energy level 21 and a conduction band energy level 22 separated by a band gap 23 for each of the layers of the PIN photodiode 10. When a hole 30 and electron 32 are separated by the absorption of a photon by the photo-active i-layer 14, the hole 30 moves in the i-layer 14 to the p-layer 12 under the influence of an electric potential gradient formed by an inherently lower energy level of the p-layer 12 for holes. Once the hole reaches the p-layer 12, the hole combines with an electron supplied by the external circuit (not illustrated). The electron 32 moves in the i-layer 14 toward the n-layer 15 under the influence of an electric potential gradient formed by the inherently lower energy level of the n-layer 15 for electrons. Electrons in the n-layer 15 enter the cathode contact (not illustrated). The drift or movement of electrons 32 and holes 30 in the i-layer 14 drives an electric current in the n-layer 15, the p-layer 12, and the external circuit.
Among the performance limitations associated with the conventional PIN photodiode is a bandwidth limitation due to the time required for the transport of holes 30 and electrons 32 within the i-layer 14. In particular, holes 30 are known to have a much slower transport velocity than that of electrons 32. The slower transport velocity of holes 30 results in a transport time for the holes 30 that is much longer than a transport time of the electrons 32. The longer hole transport time normally dominates and ultimately limits an overall response time or bandwidth of the PIN photodiode 10.
Accordingly, it would be advantageous to have a photodiode that overcomes the bandwidth limitation associated with hole transport time. Moreover, it would be advantageous if such a photodiode were similar in complexity to the PIN photodiode and provided good efficiency. Such a photodiode would solve a longstanding need in the area of photodiodes for optical networking.
The present invention provides a unipolar or uni-traveling-carrier (UTC) photodiode that employs a Schottky contact (SC) as a cathode contact. In particular, the present invention provides a metal Schottky contact directly on a collector layer or intrinsic layer (i-layer) of the photodiode. The Schottky cathode contact on the i-layer is substituted for an n-type semiconductor layer in contact with an i-layer of a conventional UTC PIN photodiode.
In an aspect of the invention, a unipolar photodiode is provided. The unipolar photodiode comprises a first semiconductor or light absorption layer in a first conduction type having a first doping concentration. The light absorption layer has a band gap energy that facilitates light absorption. The unipolar photodiode further comprises a second semiconductor or collector layer having a second doping concentration and a collector band gap energy. The light absorption layer is adjacent to and in contact with a first side of the collector layer. The collector band gap energy is greater than the light absorption band gap energy, such that the collector layer is relatively non-conducive to light absorption. The unipolar photodiode further comprises a Schottky cathode contact adjacent to and in contact with a second side of the collector layer. The second side is opposite the first side. The unipolar photodiode further comprises an anode contact indirectly interfaced to the light absorption layer.
In other aspects of the present invention, a method of detecting incident light using the unipolar photodiode and a method of constructing the unipolar photodiode of the present invention are provided.
The present invention provides a simpler UTC or unipolar photodiode construction than that of the conventional UTC photodiode, yet provides a saturation current equivalent to that of the conventional UTC photodiode. Moreover, while simpler than the conventional UTC photodiode, the SC-UTC photodiode of the present invention exhibits improved bandwidth and efficiency relative to the conventional UTC photodiode. Furthermore in some embodiments, an order of the layers within the Schottky contact unipolar photodiode of the present invention is advantageously reversed compared to that of the conventional UTC PIN photodiode facilitated, in part, by the use of the Schottky cathode contact according to the present invention. Certain embodiments of the present invention have other advantages in addition to and in lieu of the advantages described hereinabove. These and other features and advantages of the invention are detailed below with reference to the following drawings.